Far-infrared rays having a wavelength of about 3-1000 μm give energy (vibration of C—C bonds, C—O bonds, C—H bonds, etc.) to substance molecules to warm the substance. Because the far-infrared rays penetrate relatively deep into the substance, they can elevate the temperature inside the substance without elevating the surface temperature more than needed. Development has conventionally been conducted to provide fibers, clothing, bedding, etc. having excellent temperature-keeping properties utilizing such warming effect of far-infrared rays. The warming effect of far-infrared rays can be obtained by adding far-infrared-radiating components to fibers.
As a technology concerning fibers containing a component having far-infrared radiation properties, JP 3-51301 A discloses underwear having a far-infrared radiation layer containing particles of alumina, zirconia, magnesia, etc. having a far-infrared emissivity of 65% or more on average at 30° C. JP 3-190990 A discloses synthetic fibers containing far-infrared radiation particles comprising alumina, titanium and platinum. However, these far-infrared radiation particles do not have sufficiently high far-infrared radiation properties, and demand is mounting for far-infrared radiation particles having higher irradiating efficiency.
JP 2002-161429 A discloses rayon fibers formed by wet-spinning a solution in which particles of at least one metal oxide selected from the group consisting of alumina, silica, magnesia, calcium oxide and titanium dioxide, and platinum particles are dispersed. Because the particles are contained in the fibers without likelihood of detachment, wearing the underwear formed by such rayon fibers increases blood flow. However, the above far-infrared radiation particles do not have satisfactory far-infrared radiation properties. Accordingly, further improvement in the warming effect is desired.